Conventionally, heavy fuel oil has been widely used as a marine fuel. However, emission control for sulfur oxides (SOx) is increasingly strengthened in recent years, and areas in which use of the heavy fuel oil, which is sulfur-rich, as a marine fuel is acceptable are becoming narrower. Moreover, use of the heavy fuel oil as a marine fuel is expected to become completely impractical in the near future.
In such circumstances, light cycle oil (LCO), which has a low sulfur content, has been received attention as an alternative marine fuel to the heavy fuel oil (e.g., Patent Document 1). LCO refers to fractions having higher boiling points than gasoline among fractions obtained by allowing vacuum gas oil or residual oil to pass through a fluid catalytic cracker (FCC), in other words, lighter fractions (boiling point range: around 220 to 430° C.) among cracked gas oils, and is also called “cracked light gas oil”. Since LCO has a low sulfur content, use of LCO as a marine fuel advantageously leads to reduction of the amount of SOx emitted from ships.
In addition, the amount of nitrogen oxides (NOx) emitted from ships is controlled. Use of an emulsion fuel obtained by mixing a fuel oil and water as a fuel for a diesel engine is known as a method for reducing the amount of NOx emitted (Patent Document 2). An emulsion fuel generates a reduced amount of NOx formed (emitted) because the combustion temperature of the emulsion fuel is lower than that when a fuel oil is directly burned.
However, an emulsion fuel generally has low ignitability, and hence has the problem of ignition delay and the resulting frequent occurrence of abnormal vibration for diesel engines. Patent Document 3 discloses, as a solution to this problem, a diesel engine including a pilot injector configured to inject an auxiliary fuel having a high Cetane number such as marine diesel oil prior to injection of an emulsion fuel.